(1) Field of the Invention
The present invention relates to a semiconductor device using a fuse as a storage element and a test method thereof.
(2) Description of the Related Art
More and more semiconductor devices are using fuses as nonvolatile storage elements. One of the methods for disconnecting a fuse includes irradiating the fuse with laser light to blow the fuse by the optical energy of the laser light. The optical energy of the irradiating laser light is adjusted to a level sufficient to blow the fuse. However, there has been the possibility of a disconnection failure that fuses other than the one to be disconnected are also disconnected due to a maladjustment of the optical energy or irradiation position of the laser light. Failures in which unintended fuses are disconnected can also occur due to various other factors. Examples thereof include a disconnection because of stress in manufacturing steps and melting in a chemical solution or other liquid. With such semiconductor devices using fuses, it is a challenge to check that fuses to be disconnected are electrically fully disconnected, i.e., correctly disconnected.
For example, expected values when fuses are correctly disconnected may be input from outside a semiconductor device. The expected values are compared with data read from the fuses to determine whether the fuses are correctly disconnected (for example, see Japanese Patent publication No. 2012-33232). Suppose that such a semiconductor device uses a complementary fuse in which two fuses are connected in series and the connection node serves as an output terminal. If the two fuses are both disconnected, neither a power supply voltage (hereinafter, referred to as “1”) nor a ground potential (hereinafter, referred to as “0”) is supplied to the output terminal of the complementary fuse. The value read from the output terminal of the complementary fuse is not stably fixed to either “1” or “0,” and may become floating. If the data output from the complementary fuse happens to coincide with the expected value at the time of a shipping test, the semiconductor device can pass the shipping test and be shipped out. Since the complementary fuse may output data having a value different from written one afterwards, such a semiconductor device can, in the worst case, cause a problem of an apparatus on which the semiconductor device is mounted.